<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <style>
        .parent {
            /* width:100% 让当前元素的宽度和他的父元素一样宽body */
            width: 100%;
            height: 200px;
            background-color: grey;
            display: flex;
            justify-content: space-around;
            align-items: center;
        }

        .one, .two, .three {
            width: 100px;
            height: 100px;
            background-color: red;
        }
    </style>

    <div class="parent">
        <div class="one">1</div>
        <div class="two">2</div>
        <div class="three">3</div>
    </div>
</body>
</html>